1. Field of Invention
The invention relates to the field of semiconductors. More particularly, the invention is directed to group III-V nitride semiconductor films usable in blue light emitting devices.
2. Description of Related Art
The light-emitting diode is the basic component for electrically injected semiconductor lasers. A light-emitting diode is a relatively simple semiconductor device which emits light when an electric current passes through a junction of the light-emitting diode. As shown in FIG. 1, a light-emitting diode 100 includes a back-to-back sandwich of a p-type 110 semiconductor material and an n-type 120 semiconductor material, i.e., a p-n junction, characterized by a bandgap E.sub.g 130. The bandgap 130 determines the minimum energy required to excite an electron 160 from a valence band 140 to a conduction band 150, where the electron 160 becomes mobile. Conversely, the bandgap 130 also determines the energy of a photon produced when the electron 160 in the conduction band, i.e., a conduction electron, recombines with a hole 170 in the valence band 140. When current passes through the diode 100, the electrons 160 in the conduction band 150 flow across the junction from the n-type material 120, while the holes 170 from the valence band 140 flow from the p-type material 110. As a result, a significant number of the electrons 160 and the holes 170 recombine in the p-n junction, emitting light with an energy hv=E.sub.g. These semiconductor devices contain only one junction, the p-n junction, in a single material and are referred to as homojunction structures.
In order to obtain more efficient lasers, in particular, lasers that operate at room temperature, it is necessary to use multiple layers in the semiconductor structure. These devices are called heterojunction or double heterojunction lasers, depending on the number of wide bandgap layers formed.
The wavelength, and thus the color of light emitted by an LED or laser diode, depends on the bandgap E.sub.g. LEDs or laser diodes that emit light in the red region of the visible spectrum have been available since the early 1990's. There has been great difficulty in developing LEDs that emit light at shorter wavelengths. Extending LED light sources into the short-wavelength region of the spectrum, the region extending from green to violet, is desirable because LEDs can then be used to produce light in all three primary colors. Shorter-wavelength laser diodes will also permit the projection of coherent radiation to focus laser light into smaller spots. That is, in the optical diffraction limit, the size of the focused spot is proportional to the wavelength of the light. Reducing the wavelength of the emitted light allows optical information to be stored and read out at higher densities.